Linear power amplifiers are used extensively in electronic applications to drive electronic signals into low impedance loads. These amplifiers are usually designed with a single-ended output or with differential outputs, depending upon the requirements of a specific application. For example, a telephone transmission line carrying digital data according to a T1 protocol is generally driven by a linear power amplifier having differential outputs due to the large output swings and symmetry required.
The output stage of the linear power amplifier is commonly implemented with a pair of push-pull transistors. In a push-pull configuration, an upper transistor sources current to the load from a positive power supply, and a lower transistor sinks current from the load to a negative power supply. Current which flows directly from the upper transistor and through the lower transistor without flowing through the load, referred to herein as rush-through current, results in wasted power. Most of this power loss occurs during the switching phase when one transistor switches on before the other switches off. On the other extreme, if one transistor switches completely off before the other switches on, the output stage will exhibit an output deadband, which causes cross-over distortion.
In order to minimize cross-over distortion, a certain amount of rush-through current is usually allowed in the output stage to insure that at least one output transistor is on during the switching phase. As such, the output stage operates as a class-AB amplifier. The control of the rush-through current over environmental and processing variations is critical in obtaining low cross-over distortion without incurring a large power penalty. This control becomes even more critical in differential output amplifiers, which have two outputs with twice the potential for wasted power.
Therefore, it can be appreciated that circuitry which effectively controls the rush-through current in a linear power amplifier having push-pull outputs is highly desirable.